Arbuscular mycorrhizal (AM) symbioses often result in an increased resistance or decreased susceptibility of plants to soil-borne pathogens. Various mechanisms are probably involved in AM-bioprotection. AM symbioses elicit early and transient defence reactions in plants, and several defence compounds and enzymes could act as elicitors sensitizing the plants to respond, more quickly and to a greater extent, to subsequent pathogenic attacks. This work focused on the putative role of pathogenesis-related (PR) proteins in tomato plants, where a bioprotective effect was established in roots pre-colonized with the AM fungusGlomus mosseaeand post-infected with the pathogenic fungusPhytophthora parasitica. Antisera raised against tomato or tobacco proteins from PR-1, PR-2, PR-3 and PR-5 families were used to analyse PR protein expression by western blot analysis. PR protein expression was clearly enhanced byP. parasitica-infection, while only a weak increase was detected inG. mosseaecolonized roots, when compared to control plants. In mycorrhizal tomato roots post-infected withP. parasitica, PR protein level was positively related to the amount of pathogen and therefore represent a response to pathogen attack. Direct implication of PR proteins in bioprotection therefore seems questionable. Other defence proteins or other pathways could be directly involved in the bioprotection of tomato by AM fungi.